1. Field of the Invention
The present invention concerns a stepping motor controller for controlling rotational angular position and rotational speed of a stepping motor.
2. Description of the Prior Art
With the high functionalization of systems equipped with motors, motors of which noise levels and vibration levels are low, and of which rotational speed ranges are wide, are being demanded. A stepping motor has excellent properties such as a small rotational angular position error and a wide rotational speed range. However, the stepping motor is caused to make a stepping rotation by changing instantaneously excitation currents for windings at each time when an external command pulse is given. Accordingly, there have been problems that the stepping motor causes vibration and noise and that it tends to step out when the excitation current is changed.
To solve such problems, a micro-step excitation system in which an inverter of a PWM (pulse width modulation) type is used to smoothly change the excitation current for windings is being generally used.
In the micro-step excitation system being used in general, a current of a sinusoidal wave form is divided to form a plurality of stepped currents with different levels, and the stepped currents are fed to the respective motor windings with a phase difference between according to the number of phases of the motor. Accordingly, it is necessary to control a plurality of phase currents according to the number of phases of the motor. So, the construction of the controller becomes complicated with the increase of the number of phases of the motor.
To simplify the construction of the controller, it is conceived to apply an inverter controller technique that is generally used for controlling an AC servomotor, and in which rotational coordinates system is introduced, to the stepping motor controller. The stepping motor controller to which the inverter controller technique is applied is suitable for realizing a micro step drive of the motor at a high resolution, since a rotational angle can be controlled directly.
As an example of such stepping motor controller, there is a stepping motor controller disclosed in Unexamined Japanese Patent Publication No.6-225595. In this controller, each phase current in a fixed coordinates system is converted into a phase current in a d-q rotational coordinates system in which d axis is in a direction of a magnetic flux and q axis is in a direction perpendicular to the d axis, and the control of the currents are dealt with in the rotational coordinates system. An encoder is connected to a stepping motor, and closed loop control systems for current control, for velocity control and for position control are composed. Each information in the fixed coordinates system is converted into information in the d-q rotational coordinates system to carry out the position control.
Considering a current in the d axis direction to be an exciting current and a current in the q axis direction to be a torque current, a command current in the q axis is varied to generate a torque for rotating the motor, and is varied so as to make the current in the q axis direction minimum when the rotational position of the motor reaches a predetermined position.
To simplify the construction of the controller, a non-interference component is omitted and the command current is given directly on the d or q axis. In this controller, since the rotational position of the motor is controlled so as to make a position detection signal coincide with a position command, a micro step drive of the stepping motor in accordance with the resolution of a position controller and that of a position detector is possible.
In the stepping motor controller disclosed in Unexamined Japanese Patent Publication No.6-225595, the position detection signal detected by the position detector is used for controlling rotational angle of the motor. That is, a motor current is converted into a current in the d-q rotational coordinates system in which the d axis is in a direction of the magnetic flux and the q axis is in a direction perpendicular to the d axis, using the position detection signal detected by the position detector. The control in the stepping motor controller is a closed loop position control using the position detection signal detected by the position detector.
Further, a current perpendicular to the magnetic flux, that is, a current in the direction of the q axis, is controlled according to a velocity deviation to control the torque to be generated. Accordingly, the position controller and a velocity controller are required to be provided to effectuate the position control.
Thus, there is a problem that the construction of the stepping motor controller such as disclosed in Unexamined Japanese Patent Publication No.6-225595 is complicated, and consequently, expensive.
It is an object of the present invention to provide a stepping motor controller having a simple construction, low in price and having a micro step drive function with high degree of accuracy.
In order to achieve the above object, the present invention provides a stepping motor controller comprising;
current sensors that detect respective motor currents of corresponding phases,
an inverter containing a current control means for feeding stepped currents to motor windings,
a coordinate transformer that transforms the motor currents in a fixed coordinates system detected by the current sensors into currents in a first rotational coordinates system composed of a d axis being in the direction of a magnetic flux direction and a q axis being perpendicular to the d axis,
an angle calculator that transforms an external command pulse for causing the motor to make a stepping rotation into a command angle,
wherein a second rotational coordinates system composed of a dp axis and a qp axis being perpendicular to the dp axis is determined from the command angle,
a command current on the dp axis is set to be a value corresponding to a motor current amplitude,
another command current on the qp axis is set to be zero,
the motor current is projected on the dp axis and the qp axis,
the motor current is controlled so as to make a component of the motor current on the dp axis coincide with the command current on the dp axis and to make a component on the qp axis coincide with another command current on the qp axis, and
the motor is caused to make a stepping rotation by rotating the second rotational coordinates system by a predetermined angle at each time when the external command pulse is applied.
In the stepping motor controller according to the present invention, the value corresponding to the motor current amplitude is set on the dp axis as the command current, whereas zero current value is set on the qp axis as another command current. As a result, the q axis component of the current value appears due to the angle difference between a rotor angle and the command angle, and a torque is generated. Accordingly, an operation for controlling the q axis current according to the rotor angle or the generated torque is not required. Consequently, the stepping motor controller having a simple construction, low in price and having a micro step drive function with high degree of accuracy can be provided.